1. Field of the Invention
The present invention relates to communication networks, and, more particularly, to associating a station with an access point in a wireless local area networks.
2. Description of the Related Art
Wireless local area networks (WLANs) include one or more non-fixed stations (or mobile terminals) such as cell phones, notebook (laptop) computers, and hand-held computers, equipped with generally available, WLAN PC cards that enable communication among themselves as well as through a network server. An example of a WLAN network is a network that conforms to standards developed and proposed by the Institute of Electrical and Electronic Engineers (IEEE) 802.11 Committee (i.e., a network operating in accordance with one or more editions of the IEEE 802.11 standard for telecommunications and information exchange between systems).
In WLANs operating in accordance with the IEEE 802.11 standard, a network server provides support for communication between stations in different service sets (SSs), which are associated with different access points (APs). An AP is a terminal or other device that provides connectivity to other networks or service areas, and also, in some cases, coordinates communication directly between stations. An AP may be either fixed or mobile, but for most applications is fixed. Such WLAN networks allow stations to be moved within a particular service area without regard to the connections among the stations within that service area.
Most networks are organized as a series of layers, each one built upon its predecessor. The purpose of each layer is to offer services to the higher layers, shielding those layers from implementations details of lower layers. Between each pair of adjacent layers is an interface that defines those services. The International Standards Organization has developed a layered-network architecture called the Open Systems Interconnection (OSI) Reference model that has seven protocol layers: application, presentation, session, transport, network, data link, and physical. The function of the lowest level, the physical layer, is to transfer bits over a communication medium. The function of the data link layer is to partition input data into data frames and transmit the frames over the physical layer sequentially. Each data frame includes a header that contains control and sequence information for the frames.
The interface between the data link layer and the physical layer includes a medium access control (MAC) device and a physical layer signaling control device, called a PHY device. The purpose of the MAC device and the PHY device is to ensure two network stations are communicating with the correct frame format and protocol. In a WLAN, a radio is the physical device, and free space is the physical communications medium. The IEEE 802.11 standard for WLANs defines the communication protocol between the MAC device and the PHY device. For the WLAN data communication protocol, each data frame transferred between the MAC and the PHY devices has a PHY header, a MAC header, MAC data, and error checking fields. The PHY header includes a preamble that is used to indicate the presence of a signal, unique words, frame length, etc. The MAC header includes frame control, duration, source (i.e., MAC) and destination address, and data sequence number.
Typically, messages transmitted among the stations associated with the same AP (termed an extended service set, or ESS) in such WLAN networks are transmitted to the access point (AP) rather than being directly transmitted between the stations. Such centralized wireless communication provides significant advantages in terms of simplicity of the communication link as well as in power savings. One primary operation of the WLAN is the process by which a station associates (establishes a communication connection) with an AP. When a station associates with an AP, a basic service set (BSS) is formed. Association is typically initiated by the station, and may occur in either an active mode or a passive mode. In passive mode, the station listens for periodic control messages, called beacons, sent by an AP that indicate the service set identification (SSID) of the AP. In active mode, the station sends a probe request, and each AP receiving the probe request transmits a probe response to the station with its SSID.
In the IEEE 802.11 standard, the station has signal quality information pertaining only to the link from the AP to the station. This signal quality information, such as received power or signal-to-noise ratio (SNR), is measured by the station. The station then associates to the AP based on the strongest received power level. The station associates to the AP using one or more control messages to, and with appropriate acknowledgment messages from, the AP.